This invention relates generally to optical interferometers of the type used to measure the spectrum of a narrow-band light source and, more particularly, to a method and apparatus for maintaining the separation of the reflecting plates at a desired relationship to a reference optical wavelength.
Spectral characteristics of light sources can be conveniently measured with optical interferometers. Two of such devices are the Fabry-Perot interferometer (multiple beam interference) and the Michelson interferometer (two beam in interference). In these interferometers, when the mirror separation or optical path length is changed in some way, the radiant power detected on the optical axis changes especially in the case of sources with interesting spectra. A substantial body of scientific and technological work relates to inferring the spectrum of the incident radiation (radiant power in a spectral interval vs. wave-length or frequency) from the interferogram (radiant power vs. optical path length).
In order to accurately measure an interferogram, the optical path length difference between optical elements of the interferometer (usually mirrors) must be known; and the angular alignment of the optical elements precisely maintained. One method of accurate positioning of the interferometer mirrors is mechanical or passive stabilization. Scan distance is determined by a lead screw or fixed spacer of some sort and very careful mechanical design is used to reduce the effects of creep, vibration, thermal changes, and other sources of dimensional error. Mirror alignment is also relatively stabilized by careful design and, in the case of the Michelson interferometers, retroreflectors of the "cat's eye" or cube corner type are sometimes used.
A second method for accurate positioning of optical elements is that of active stabilization. Active stabilization of interferometers is based on various techniques for sensing mirror separation coupled with electro-mechanical adjustment of mirror position by means of piezoelectric translators or motorized screws or wedges. One method of sensing spacing and parallelism of interferometer elements is by means of capacitance micrometers. Changes in distance, caused by physical factors, between capacitor plates, located on the mirror and some fixed point, respectively, are sensed. This technique has been applied, for example, to a Fabry-Perot interferometer manufactured by Queensgate Instruments Ltd., Franklin Road, London, England SE20 8MW. There is no fundamental relationship between the capacitance of the sensors and the wavelength of the incident spectrum so the relationship must be established empirically.
Another active stabilization method is to use the interferogram itself as a measure of separation and alignment. This method is feasible only if the spectrum contains a single sharp, distinct peak. The interferometer plate separation is changed by a small amount, and a logic circuit determines if the contrast of the interferogram has increased or decreased and so adjusts the direction of the subsequent change. A Fabry-Perot interferometer using this method is available from Burleigh Instruments, Inc., Fishers, N.Y. 14453. An additional disadvantage is that it is difficult to make quantitative spectral intensity measurements when the interferogram contrast is being changed for stabilization sensing.
Scanning Michelson interferometers are available where the interference fringes from a reference laser (usually HeNe) are counted to determine the optical path difference at any point of the scan. A reference interferometer is attached to the main interferometer such that the main and reference mirrors move together. Devices of this type are available from Nicolet Instrument Corp., 5225 Verona Road, Madison, Wis. 53711, Eocom Corp., 19722 Jamboree Boulevard, Irvine, Calif. 92664, and Bomem Inc., 910 Place Dufor, Vanier, Quebec, Canada. In these instruments the reference interferometers are used to measure the scan rather to stabilize the units at a fixed spacing.
Many very high resolution spectroscopy applications involve scattering from a laser source. Examples include Brillouin and Rayleigh spectroscopy in solids, liquids, and gases. Often these spectra are relatively simple, and many important features of the spectra can be inferred from one or a few interferometers operating at fixed optical path differences as specialized filters. In order to be useful, the intereferometers must have optical path differences set in a precise relationship to the wavelength of the laser source. One way to achieve this relationship, which is usually to operate at an extremum (a maximum or minimum) of the interferogram, is to tune the laser wavelength to match some multiple of the interferometer path difference as that path length difference changes from outside influences. A. Olsson et al. in Applied Optics, Vol. 19, No. 12, 1897 (1980) describe a device to tune a laser to an extremum of a passive interferometer by "dithering" the laser frequency around a central valve. This instrument is restricted to operate with a single interferometer and requires a tunable laser to work. Additional limitations of the instrument are that there is no provision for active alignment stabilization of the interferometer and that it must rely upon passive techniques for mirror tilt correction. A further disadvantage is that it is difficult to make quantitative transmission measurements when the interferometer transmission and laser wavelengths are being changed rapidly (dithered).